Where has a 7 yo got the idea of threatening someone with a knife? Is he copying something? Can you limit what he sees on tv and on-line? Have you moved the knife block to an out of reach cupboard. Cutlery drawer & Scissors too.
Can you get him involved in more sport type activities? Basically wear him out cycling & playing football. Limit his screen time, and his sugar intake / fizzy drinks. Lots of fresh air & back to basics, calm routine, earlier bed time, & some 1:1 time with you so he feels he has your attention.
It’s a lot of work. Do you have family who will help you? His dad or an uncle/older brother who will take him to football practice?
Some councils have support staff who will help. Have you discussed with his teacher? Can you ask the school for help?
i have a 10yo and he definitely needs a male role model. He does karate twice a week which is a good outlet for any aggression and the Sensei is very supportive.